In the prior art, in order to improve power efficiency by reducing power consumption, switching power supplies including semiconductor devices for switching power supply control have been widely used as the power supplies of home appliances such as household electrical appliances. The semiconductor devices control output voltages by the switching operations of semiconductors (switching elements such as transistors).
However, switching power supplies generate high switching noise in response to switching operations for turning on/off switching elements, so that the switching power supplies acting as noise sources may cause problems such as a malfunction and a failure in other electronic equipment. Thus switching power supplies with reduced switching noise have been demanded.
Further, in order to increase power supply efficiency, switching power supplies with reduced switching loss have been also demanded.
In response to these demands, for example, a switching power supply is available which reduces switching noise and switching loss by using a ringing choke converter (RCC) pseudo-resonance control system for soft switching.
The switching power supply detects proper timing from a ringing voltage which is generated after a switching element is turned off and the passage of secondary current through the secondary winding of a transformer is stopped, and then the switching power supply turns on the switching element, so that soft switching is achieved.
A device for detecting turn-on timing is disclosed in, for example, Japanese Patent Laid-Open No. 2005-287260. In a switching power supply disclosed in this document, as shown in FIG. 9, a turn-on detection circuit 7 detects a time when a voltage induced to an auxiliary winding 110c of a transformer 110 is switched from a positive voltage to a negative voltage by the switching operation of a switching element 1, through resistors 143 and 144 connected to the auxiliary winding 110c. Thus the switching element 1 is controlled to be turned on when a voltage applied to the switching element 1 decreases.
Further, a device for detecting turn-on timing is disclosed in, for example, National Publication of International Patent Application No. 2003-524359. In a switching power supply disclosed in this document, as shown in FIG. 10, a switching element 1 is controlled to be turned on when the time derivative of a voltage applied to the switching element 1 is equal to a certain reference value not larger than 0 and the voltage applied to the switching element 1 is smaller than an input voltage, that is, the voltage applied to the switching element 1 reaches the minimum value as a result of a comparison made by a comparator circuit 9.
However, in the switching power supply of the prior art disclosed in Japanese Patent Laid-Open No. 2005-287260, the turn-on timing of the switching element is detected in a pseudo manner from the voltage induced to the auxiliary winding, so that the accuracy of detection is lower than the case where the voltage applied to the switching element is directly monitored. Since the switching element is turned on when the voltage reaches the minimum value, an adjustment has to be made by an external component such as a capacitor. Moreover, an external terminal to be connected to the auxiliary winding is necessary, thereby increasing the number of external components and the cost.
In the switching power supply of the prior art disclosed in National Publication of International Patent Application No. 2003-524359, the voltage applied to the switching element is directly monitored and thus the switching element can be more easily turned on at the minimum value as compared with the switching power supply disclosed in Japanese Patent Laid-Open No. 2005-287260. However, the circuit for detecting the turn-on timing of the switching element has to be resistant to a high voltage applied to the switching element.
Generally, a voltage applied to a switching element immediately after the turn-off of the switching element fluctuates due to the leakage inductance of a transformer, the parasitic capacitance of the switching element, and so on. Thus in the switching power supply disclosed in National Publication of International Patent Application No. 2003-524359, the switching element is controlled to be turned on only when the voltage applied to the switching element is lower than the input voltage, so that the turn-on timing of the switching element is not erroneously detected.
Thus in a semiconductor device for switching power supply control disclosed in National Publication of International Patent Application No. 2003-524359, a comparator connected to the switching element and an input line is provided, so that an external terminal to be connected to the input line is necessary. The comparator also has to be resistant to a high voltage. The comparator made up of a high-withstand voltage element increases a chip area and the number of manufacturing steps, resulting in higher cost.